The dairy cattle genome has been significantly restructured over the past 30 years due to intensive breeding effort selecting for production traits, including high quality milk and high and sustained productivity. However, while those efforts led to dramatic improvement of productivity, there has been significant reproductive deterioration in high-producing dairy cows, which in turn has caused substantial economic loss in the dairy cattle industry (Lucy, 2007, Fertility in high-producing dairy cows: reasons for decline and corrective strategies for sustainable improvement. Society of Reproduction and Fertility Supplement. 64 237-254). Key factors contributing to decreasing fertility of dairy cow are low fertilization rates and decreased embryonic survival.
Fertility is a complex trait that comprises developmental stages such as combining sperm and egg to form a zygote, compaction of embryo cells to form a morula, establishment of the blastocyst, attachment of the embryo to the uterus, and fetal development (Amann and DeJarnette, 2012). This complexity makes accurate prediction of successful pregnancy difficult, as aberrant development of sperm, oocyte, embryo, or fetus all would lead to conception failure. Conception rate in dairy cattle is about 40%, and only 50% of the fertilized eggs produce viable embryos (Santos et al., 2004). The decline in reproductive performance in cattle over the past few decades (Dobson et al., 2007) has been ascribed primarily to fertilization failure and early embryonic loss (Santos et al., 2004).
Previous studies have shown that genetic makeup of an individual plays crucial roles in embryonic development and reproductive success (Weigel, 2006; Shook, 2006). Although a male and female parent each contributes half of its genetic material to the new zygote and both are necessary for embryo development, it is not obvious whether or not this contribution is equally important to pregnancy success. For example, it is well established that the paternal genome supports growth of extra-embryonic tissues while the maternal genome fosters development of the embryo proper (Barton et al., 1984). After fertilization, the development of an embryo is controlled by maternal genomic information that is accumulated during oogenesis (Telford et al., 1990). It is only at the 8-cell stage in the bovine embryo that the embryonic genome activates and the embryo switches to transcribing its own RNA (Memili and First, 2000).
Despite that most breeding schemes in cattle are focused on the selection of elite bulls using progeny testing or genomic selection, and that some semen traits (e.g., sperm motility and percentage of abnormal sperm) show moderate to high heritabilities (Druet et al., 2009), most fertility studies in cattle have focused on the maternal contribution, and the paternal contribution to reproductive performance has not been thoroughly investigated, and only a few studies have been reported in the literature (Feugang et al., 2009; Khatib et al., 2010; Peñagaricano et al., 2012). Therefore, characterization of bull fertility markers is both feasible and highly desirable, and the deployment of these markers in cattle breeding would lead to improved reproductive performance in cattle.
A recent comparative genomics study has characterized many genes involved in the control of spermatogenesis that were highly conserved from fly to human (Bonilla and Xu, 2008). Some of these genes were reported to be crucial for human fertility. However, it is not known whether or not these spermatogenesis genes play important roles in the fertility of bulls.